<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>my css2</title>
    <style>
        html {
            font-size: 16px;
        }
        .c1 {
            font-size: 2rem;
            color: rgb(0, 0, 100);
        }
        .c2 {
            color: #990000;
            font-size: 2em;
        }
        .c3 {
            background-color: gray;
            width: 60%;
            color: darkorchid;
        }
    </style>
</head>
<body>
<div class="c3">
    hello world!
</div>
<div class="c1">
    这是c1
    <div class="c2">
        这是c2
    </div>
</div>
</body>
</html>